The major objective of the proposed work is to elucidate the molecular mechanisms governing cell cycle dependent gene expression. The primary focus is to understand the regulation of histone gene expression during the HeLa cell cycle. The proposed studies will be pursued according to the following course. First, additional alleles of the human histone genes will be cloned and fully characterized in order to identify alleles encoding variant core histones, and histone H1 genes, which may be constitutitively expressed during the cell cycle, or, alternatively, expressed in a tissue specific manner. Attention will be given to the gross genomic organization of the human histone genes and to potential relationships between their topological arrangement and function. Second, in vitro studies of both the transcriptional and post-transcriptional mechanisms which operate to regulate individual histone mRNAs (in progress) will be conducted in soluble extracts of synchronized cell populations. This will include the preparation and analysis of mutant templates in order to assess the nucleotide sequences necessary for these processes. Third, in vivo studies of both the rate of synthesis and half life of histone mRNAs under a variety of growth conditions (especially when macromolecular synthesis is blocked using specific inhibitors) will be done to gain insight into these mechanisms. These studies will be extended to other more normal cell types to assess the generality of these processes. Fourth, additional cell cycle regulated genes will be cloned, and their expression studied using all of the above approaches. These studies should provide fundamental insights into the specific mechanisms for regulating cell cycle dependent gene expression at the transcriptional and post-transcriptional levels. It is hoped that they may also provide an avenue towards understanding the mechanisms controlling the mammalian cell cycle.